THE SOILS OF FLORIDA. 
17 
The hard rock phosphate of Columbia, Alachua, Marion, Levy, 
Pasco and Hernando Counties is found resting upon the Vicksburg 
Limestone, all formations later than the Vicksburg having disinte¬ 
grated or nearly so, their residue forming the matrix and the phos 
phoric acid which they contained having become segregated to form 
the phosphate boulders. To the northeast of this phosphate field is 
found the uneroded plateau, beneath which, on this hypothesis no 
deposits of high grade rock phosphate are to be expected. 
A limestone sub-structure is favorable to the chemical action 
which results in the formation of phosphate boulders. In the non- 
limestone section drainage is largely by surface streams. Under 
these conditions the phosphoric acid gathered into solution in small 
amounts from various sources throughout the soil is carried directly 
through streams to the ocean. In the limestone sections, on the con¬ 
trary, the drainage is chiefly subterranean, and the rainfall after 
passing through the soil and underlying material and thus gathering 
up more or less phosphoric acid, passes into the limestone. Within 
the earth, and especially at and below the underground water level, 
the phosphoric acid is again thrown out of solution, thus forming 
the phosphate boulders. This hypothesis of the origin of the hard 
rock phosphate involves only those natural processes which are con 
stantly in operation. It does not postulate a chain of islands the 
existence of which has not been demonstrated. Nor is it necessary 
to invoke the aid of bird rookeries, although these were doubtless a 
feature of the past as of the present, birds contributing then as now 
to the phosphoric acid supply of the soil. No one questions that the 
flint boulders which lie alongside the phosphate boulders are formed 
by chemical segregation of silica from various sources. An identical 
chemical process accounts for the phosphate boulders. 
PLEISTOCENE. 
Owing to the local character of the Pleistocene deposits much 
difficulty is experienced in describing the formations of this age. 
Local shell bearing deposits of Pleistocene age are found at many 
localities both along the coast and for some distance inland, par 
ticularly along the St. Johns River and in the Kissimmee River Val 
ley. The limited extent'of these deposits and their local character 
make it inadvisable to differentiate formations among the deposits. 
Some of the localities at which marine Pleistocene shell marls have 
been located are the following: on the Gulf coast; North Creek, a 
tributary to Little Sarasota Bay in Manatee County; Labelle, on 
the Caloosahatchee River; and on Six-Mile Creek near Tampa; and 
2—Gr 
